1. Field of the Invention
This invention relates to a chemically bondable foundry sand comprising a mixture of sand and binder. The term foundry sand is intended to relate to granular material from which moulds and cores are made, for example, silica sand, zircon sand, chromite sand, olivine sand, silicon carbide in granular form, iron and steel shot, salt (sodium chloride in dry granular form), chamotte (and other aluminosilicate type granulated products), and all such granular material is referred to herein as foundry sand.
2. Description of the Prior Art
There are available today two main types of chemical binder systems for foundry sand. One of the two main types is a silicate binder such as sodium silicate which whilst having good environmental properties is not easily reclaimable and does not break down well after casting so that cores are difficult to remove. The other main type is constituted by organic resins, including phenolic and furane types. These are generally less pleasant to handle and are often rather more costly than those of the first type but have the advantage that they break down significantly on casting, making de-coring easy, and are relatively easily reclaimed by various methods in which the remaining resin is either abraded or burned off the sand grains.
All these binders are mixed with clean, or washed sand, and are caused to harden, either by heat or chemical reaction with a gas or other chemical additive to the mixture, so forming a strong mould or core.
Naturally, strength depends on the amount of binder but the amount is restricted by foundry technologists in an effort to reduce costs; reduce striking to pattern-work; reduce contraction resistance of the casting, so reducing hot tearing problems, and improve knock-out.
Thus, strength is generally reduced to a minimum level at which the regular breakage of cores becomes a significant problem. This involves extra cost in the core since many cores never reach the mould in one piece and involves even more expense if breakage occurs in the mould during or after closure, or during casting.
Although chemical binder systems have now been used by the foundry industry for over two decades, there has been no good solution to the above problem. The usual practice has been to resort to wires or nails which have to be laid by hand into the core during manufacture, precluding the use of automatic core blowing machines.
It is accordingly an object of the present invention to provide a chemically bondable foundry sand whereby the above mentioned problems are overcome or are reduced.